The principle metallic components charged into basic oxygen furnaces, usually encompassing more than 95 percent of the total metallic charge, are molten pig iron (hot metal) and steel scrap. The temperature, composition, and quantity of hot metal available to the basic oxygen melt shop, however, rigidly governs the amount of scrap that can be melted and thereby determines the amount of raw steel produced. This limitation on the basic oxygen furnace is imposed by the thermochemistry of the process in which the heat available to melt scrap is provided by the sensible heat of the hot metal plus the heat released by the oxidation of iron, carbon, silicon, manganese, and phosphorus during the injection of a stream of high purity oxygen into the molten bath.
With the composition and temperature of hot metal commonly used in basic oxygen furnaces, the charge mixture is thermally balanced with a mix of 70 percent molten pig iron and 30 percent scrap. Unfortunately, this charge mix approximates the amount of steel scrap generated by primary and finishing rolling mills and thereby rigidly restricts total raw steel production to an amount determined by the 70 percent molten pig iron charge. The average charge mixture for basic oxygen furnaces is 1625 lb. of hot metal and 675 lb. of scrap per ton of raw steel produced.
Operators of basic oxygen furnaces have attempted to increase the scrap melting capability of the process by preheating the scrap charge with fossil fuels, both within and exterior to the vessel, or by adding powerful manufactured fuels such as calcium carbide, silicon carbide, or 50% ferrosilicon.
Typical of some of the procedures which have been employed in the prior art are the following more recent U.S. Patents:
U.S. Pat. No. 3,535,106 to D'Entremont discloses a process whereby scrap is preheated using natural gas and air and then charged into a basic oxygen vessel charged with solid fuels.
U.S. Pat. No. 3,514,279 to Jensen describes a method for preheating scrap prior to charging the scrap into an electric arc steelmaking furnace.
U.S. Pat. No. 3,399,992 to Ciochetto is concerned with using a plurality of furnaces in which scrap is preheated to 1400.degree.- 1500.degree. F. prior to charging the scrap into a basic oxygen furnace. This process permits only a modest increase in the amount of scrap used and requires the use of fossil fuels.
U.S. Pat. No. 3,223,521 to Stone describes a device for preheating scrap to about 2000.degree. F. without excessive oxidation of the scrap during heating and subsequent charging of the preheated scrap to a basic oxygen furnace.
U.S. Pat. No. 3,180,724 to Brooke discloses using a chamber in which cold metallic materials such as scrap, briquettes, sponge are preheated prior to being transferred into a conventional melting and refining furnace.
These procedures of the prior art, however, all involve fundamental inefficiencies and do not permit significant increases in the relative amount of scrap which can be employed in the basic oxygen process.
For example, by the addition of roughly 20 lbs. of calcium carbide, silicon carbide, or 50% ferrosilicon the charge mixture can be altered to melt an additional 150 lbs. of scrap and thereby displace a nearly equal amount of hot metal. However, the use of such supplementary fuels, also, increases the consumption of oxygen, burnt lime and dolomite (except for calcium carbide which essentially equally displaces burnt lime), fluorspar, and measurably lengthens the time to produce a batch of steel.
Further, the alternative for increasing scrap consumption by preheating the scrap within the vessel substantially increases the consumption of oxygen, inefficiently consumes fossil fuels, and severely lengthens the time required for processing. In addition, because of the low specific heat of solid steel, (0.145 Btu/lb/.degree. F) it is necessary to preheat the entire scrap charge to roughly 800.degree. F to accomplish an increase of 100 lb. of scrap in the charge mixture. Accordingly, the charge mix ratio would change from the normal 70/30 ratio of hot metal/scrap to a new level of 68/32. To accomplish this minor alteration of the hot metal/scrap ratio would require the consumption of fossil fuel equivalent to that of 200 cu. ft. of natural gas plus 400 cu. ft. additional oxygen per ton of raw steel produced.